See, for example, http://properdiscord.com/2014/07/22/scott-timberg-ive-got-a-...
There are many other similar posts, all full of facts, on this blog. Alex Ross himself has written on the subject.
Now, name a modern, orchestra composer who's work you've listened to (excluding motion picture soundtracks)?
Now, imagine if rock music stopped at Pink Floyd's The Wall.
Ticket sales don't lie. Symphonies are shutting down all over the country.
Alan Hovhaness: https://www.youtube.com/watch?v=nG_gAMwGG-Q
Some symphonies are shutting down, but the Chicago Symphony just announced increases in ticket sales and attendance. Excitement over my local orchestra, the Seattle Symphony, is increased in recent years since they hired a new music director and commissioned a Pulitzer-prize winning piece by John Luther Adams.
For me, I think my concern about modern composers is that I associate them with highly dissonant or atonal music... I realize those are problematic descriptions, but I don't know the right terminology. Now, I do like atonal music to some degree -- I play the shakuhachi; perhaps honkyoku could be lumped into "atonal" somehow -- but encountering it at a concert where it wasn't the main draw for me is not always the most exciting experience, especially if I'm bringing someone who isn't into classical music and now they have to hear something that sounds like violins dying for 20 minutes (disclaimer two: I love Takemitsu's November Steps, which has violins that sound like they're dying).
I'm not saying that older music is objectively better or something like that, but I think it's not hard to argue that for many people it's a bit more approachable. As such, I think that artists who can write works that span between the old and new, such as in the case of Mason Bates' piece, will be critical in bringing in new audience members.
Whenever I would go to a chamber music event or an orchestral performance there will generally be two sets of people there - the old school bougie folk and a handful of conservatory students - the casual audience member is far and few between. Orchestras will have a better chance getting an audience by performing pops or some program full of video game music.
Still, going to the symphony hall, being part of that scene, this might retain it's importance for the upper class / professionals, and this alone could be enough to continue to support more than a handful of orchestras around the United States.
* And the only reason I can is because I studied composition and theory at University.
Why are we excluding motion picture soundtracks?